These functional fluids are used in hydraulic systems, such as, for example, gas turbines and steam turbines, and in the turbine control as lubricants or also as hydraulic media. Often, in addition to needing to be suitable as lubricant media or hydraulic media, the fluids must be poorly flammable. To achieve proper operation of the respective hydraulic system, the fluid used, during the entire time that it circulates, for example, in a fluid circuit of the hydraulic system, must have a very high quality. In particular, impurities of any kind that accompany especially the degeneration of the fluid in use are to be allowed only up to a maximum amount, which amount must generally be set very low. Therefore, in hydraulic systems that do not provide cleaning of the fluid, when a maximum allowable contamination is reached, the complete replacement of the used fluid by a new fluid is necessary. This replacement is, on the one hand, very labor-intensive in most hydraulic systems, and, on the other hand, the indicated fluids are very expensive, especially when they are endowed with special properties such as being poorly flammable and the like. This disadvantage of replacement prompts a legitimate interest in prolonging the service life of the fluid used. The cleaning of the fluid in particular lends itself for accomplishing this purpose.
Filter apparatus having a filter bed of anionic and cationic ion-exchange resins (U.S. Pat. Nos. 3,708,508 and 4,741,857) are used to remove corrosive acids from functional fluids, especially poorly flammable, phosphate ester-based hydraulic fluids (HFD-R). Aging of these fluids is due to hydrolysis, in which the decomposition products are, for instance, corrosive fluids. The neutralization number Nz, in English also referred to as “TAN” (Total Acid Number), and the content of metal soaps can be reduced by special ion-exchange resins. In addition to this cleaning of the fluids, continuous dewatering of the fluid is often carried out as well. The desired value of the neutralization number of the cleaned fluid is Nz≦0.1 mg KOH/gfluid. For practical use of the fluids in hydraulic systems, values of the neutralization number Nz of up to 1 mg KOH per gram of fluid can be easily achieved.
EP 0 696 311 B1 discloses a method for treatment of a nonaqueous functional fluid, selected from among phosphate esters, mineral oils, and carboxylate esters, in which a portion of the fluid quantity makes contact with an anionic ion-exchange resin in the presence of enough water so that the acid content of the fluid can be reduced. Then, in the known solution, the removal of water from the fluid by vacuum dewatering and finally the combination of the treated fluid with the bulk of the remaining fluid take place (in situ).
U.S. Pat. No. 5,661,117 B1 discloses a method for regeneration of phosphate ester lubricating fluids in which the lubricating fluid circulates in a mechanical system and is contaminated with metallic material and phosphorus-based acids. Cleaning leads to a new fluid quality with a TAN (Total Acid Number) of 0.03. The method includes essentially the following steps:                Preparation of a source of a phosphate ester lubricant that circulates in a mechanical apparatus and that has at least one metal from the group consisting of aluminum, chromium, tin, iron, sodium, calcium, magnesium and silicon, and has phosphorus-based acids;        Preparation of a source of anionic ion-exchange resin in a cartridge in fluid communication with the apparatus, with the resin having a liquid content of at least roughly 50%;        Introduction of the phosphate ester lubricating fluid contaminated with the metals and the acids for purposes of contact with the resin; and        Removal of the metallic compounds and the acids, using the resin by replacing the filter with a periodicity of up to 27 months, when there is a rise in the TAN of roughly 0.07 to make available a reusable lubricating fluid that is essentially free of impurities in new quality and that has a Total Acid Number of 0.03.        